1. Field of the Invention
The present invention relates to a CMOS (Complementary Metal Oxide Silicon) image sensor and a method for sensing an image using the same.
2. Description of the Related Art
A photo detector detects light, converts the detected light into an electrical signal, and outputs an image signal. For example, the photo detector may be included in a CCD (Charge Coupled Device) or a CMOS image sensor.
The CCD includes a light-sensitive element such as a photo diode, a charge transmission element and a signal output element. The photo diode detects light and generates a charge signal (or a photo charge) that is indicative of the amount of light received by the photo diode. The charge transmission element transmits the charge signal generated from the photo diode without signal loss to the signal output element. The signal output element accumulates the charge signal, and outputs an analog voltage signal in proportion to the quantity of the charge signal.
The CCD sequentially transmits the charge signal to neighboring pixels, but does not randomly access the pixels.
A CMOS image sensor has a few advantages over the CCD. In particular, the fabrication process of the CMOS image sensor is simpler than that of the CCD. In addition the CMOS image sensor employs a correlated double sampling circuit to greatly reduce a reset noise caused by resetting the charges accumulated from the photo diodes.
The correlated double sampling circuit samples a reset voltage of a pixel, and then samples a signal voltage. An output of the correlated double sampling circuit equals the difference between the reset voltage and the signal voltage. Thus, the correlated double sampling circuit may reduce fixed pattern noises due to threshold voltage differences of the transistors in pixels, and the correlated double sampling circuit reduces the reset noise due to the reset voltage differences.
One example of a pixel in the CMOS image sensor has a photo diode and four transistors. The four transistors function to transfer charge from the photo diode to the correlated double sampling circuit and to reset the accumulated charge. FIG. 1 illustrates an example of a CMOS image sensor having a photo diode and four transistor structure. As shown, a plurality of row lines 115 cross a plurality of column lines 113. At respective crossing, pixels 101 having a photo diode and four transistor structure are formed. As further shown, reset lines 117 provide reset signals Rs for causing the pixels 101 to reset their charges, and selection lines 119 provide selection signals RSEL for causing pixels to transfer charges to an associated column line 113.
The CMOS image sensor having pixels of the four-transistor structure has an advantage that noise is reduced, but has a disadvantage that the fill factor of the pixel is low. In other words, the area occupied by the photo diode in one pixel is relatively reduced since the pixel includes four transistors. An increased number of pixels and a decreased area occupied by a unit pixel may lead to a CMOS image sensor of high resolution. A low fill factor leads to a decrease in the area occupied by the photo diode. This decreased area occupied by the photo diode reduces the quantity of the electron-hole pairs generated by light incident on the photo diode, and thus the quantum efficiency (Q.E.) of the CMOS image sensor decreases. Therefore, the decreased quantum efficiency of the CMOS image sensor deteriorates the sensitivity of the CMOS image sensor.